QuickTime is Apple's own proprietory video format and QuickTime player is the official player for it.
As you would expect from an Apple product, Quicktime is extremely slick to use to the point of sometimes "hiding" features that you would expect to be more readily available - such as creating playlists for example. When it comes to performing basic functions such as playback, it's very easy to use although launching it is still a little slow. If you want to really get the most out of QuickTime however, you need to upgrade to the Pro version which allows you to do much more such as edit and convert video files.
QuickTime is designed to play Apple's MOV video format although but beyond this on Windows, it isn't very flexible. There are other players such as VLC media player which play almost any format and are much quicker to launch.
QuickTime is Apple's flagship media player although there are many other free players out there for Windows that are much more powerful
- Includes changes that increase reliability, improve compatibility and address security.